The ability of infrared sensor systems to image targets is often limited by thermal background noise resulting from self-emission of warm surfaces within the sensor's optical train. Secondary mirrors support struts, which are generally necessary in most centrally-obscured reflective optical systems, are often a source of thermal background noise. Because these struts are in the sensor's field-of-view, thermal emission from the struts tends to propagate down the optical path to the sensor.
Conventionally, struts have been painted black, which render them highly emissive, and therefore susceptible to contribute significantly to thermal background. Newer strut designs, however, generally employ highly reflective continuous stair step designs to view cold surfaces of the sensor system; for example, the sensor and the dewar portion of the sensor system.
While strut designs using a highly reflective continuous stair step design are more effective than painted black struts or purposes of viewing cold surfaces, there are sometimes multiple optical paths by which the sensor may observe thermal noise produced by the struts. These multiple pathways usually involve multiple reflections between the primary mirror, the strut surface facing the sensor, and the secondary mirror. Typically, the strut surface is not cooled and is generally disposed within the field of view of the image path. As a consequence, the strut surface may be substantially directly seen by the sensor, and reflections from the strut surface may cause the sensor to view warm parts of the system. Considerable thermal background noise may therefore be viewed. Accordingly, there exists a need for a strut design that overcomes these and other deficiencies of the prior art.